A. Field of the Invention
The present invention relates to the coating of fibers with liquid-applied organic materials which are thereafter cured to form solid protective organic coatings on the fibers, and more particularly, to the coating of glass optical waveguide fibers.
Fibers are frequently provided with a coating for protection against mechanical damage, for insulation, for identification and for other purposes. An optical fiber, for example, is drawn from a source such as a crucible or preform and then passes successively through a cooling tube, one or more coating means, and a draw mechanism such as a tractor; it is then wound on a spool. A diameter measuring means is located between the source and the cooling tube; another diameter measuring means is optionally located after the coating means. The drawn optical fiber should be provided with a protective coating prior to its coming into contact with any other surface such as the draw tractor.
An optical fiber is coated by drawing it through a container of liquid coating material and then through a sizing die where excess coating liquid is removed from the fiber. As the fiber traverses downwardly through the surface of the coating material, it draws the surface down and forms a meniscus. As draw speed increases, the meniscus penetrates deeper into the liquid coating material. Above a certain draw speed, the surface of the coating material is essentially tangent to the fiber surface at the point where the two surfaces contact, and incipient air entrainment occurs. Below a certain draw speed, this incipient air that is entrained into the coating dissolves and is not seen in the coating. As the draw speed increases, more air is entrained, until a limit is reached, and visible bubbles are seen in the fiber coating. Oxygen in the entrained air inhibits curing of the coating. Moreover, excessive bubbles are considered to be a visual defect and, when sufficiently numerous or sufficiently large, may induce microbending loss in the optical fiber.
B. Description of the Prior Art
U.S. Pat. No. 4,704,307 (Jochem et al.) and C. M. G. Jochem et al., "High-Speed Bubble-Free Coating of Optical Fibers on a Short Drawing Tower", Proc. IOOC-ECOC'85 (Venice, Italy, Oct. 1-4, 1985), pp. 515-518 relate to a method and apparatus for providing a bubble-free coating on optical fibers that are drawn at speeds higher than those at which similar coatings can be drawn when air surrounds the fiber at the point of entry into the liquid coating material. C. M. G. Jochem et al. teach that when the area in which the fiber and coating material meet is surrounded by a purge gas having a comparatively low kinematic viscosity (lower than that of air), the risk of bubble formation is reduced in that such a gas is less readily taken along by the fiber. Argon, Xenon and CCl.sub.2 F.sub.2 are said to be suitable purge gases.
U.S. Pat. No. 4,792,347 (Deneka et al.), which is incorporated herein by reference, teaches a method for applying a protective coating to an optical fiber by applying a curable liquid coating material thereto and subsequently curing the liquid coating to form a protective plastic layer. Air is purged or displaced from the surface of the fiber prior to the application of the liquid coating material thereto by replacing air adjacent the fiber surface with a purge gas that preferably exhibits high solubility in the liquid coating material and resists bubble formation in the liquid coating layer as it is formed. The Deneka et al. patent states that suitable purge gases are nitrogen, carbon dioxide, and the Group VIII or so-called noble gases, e.g., xenon, neon, argon or the like, and that chemically inert halocarbon gases or vapors thereof, such as chloroform, Freon.RTM. halocarbons, or other chlorine- or fluorine-substituted hydrocarbons may also be considered. All of the purge gases mentioned in U.S. Pat. No. 4,792,347 displace the oxygen that is present in air and hence reduce its inhibiting effect on cure. For example, nitrogen does not reduce bubbles in the applied coating, although it is a safe and inexpensive gas to displace oxygen from above the coating. Argon functions in a similar manner.
Helium is not mentioned in U.S. Pat. No. 4,792,347 as one of the noble gases which would be suitable for preventing bubbles in the applied coating. Helium was not considered to be a good candidate for a purge gas since U.S. Pat. No. 4,704,307 and the aforementioned C. M. G. Jochem et al. publication teach that the kinematic viscosity of a purge gas should be sufficiently low, i.e. lower than that of air, and helium is listed in Table 2 (page 516) of the C. M. G. Jochem et al. publication as having a kinematic viscosity of 110.0.times.10.sup.-6 which is 7.43 times greater than that of air. Moreover, Table 2 and the text of the Jochem et al. publication lead one to believe that helium was tried experimentally and (independent of the theoretical explanation) did not reduce bubbles in practice.